This invention relates to an apparatus for detecting the passage of multiple superposed sheets along a feed path. The invention has application, for example, to an apparatus for detecting the passage of superposed currency notes in a cash dispensing mechanism of an automated teller machine (ATM).
In a cash dispensing mechanism, it is important to provide a simple and reliable means for detecting when a currency note has become superposed on another in a path of travel from a currency supply means to a note exit slot, since such superpositioning may produce an undesirable result such as the dispensing of an excessive amount of money. For convenience, two or more sheets or notes which have become disposed in a superposed relationship will hereinafter be referred to as a multiple sheet or a multiple note.
From EP-B-0344938 there is known an apparatus for detecting multiple sheets. This apparatus includes first and second cooperating rollers between which sheets pass as they are fed along a feed path, the first roller having a fixed axis of rotation, and the second roller being resiliently urged towards the first roller so as to enable it to be moved away from the first roller as a single or multiple sheet passes between the rollers. A voltage generating means associated with the second roller produces an output voltage which varies linearly with movement of the second roller towards or away from the first roller, and this output voltage is applied to an analog-to-digital (A/D) converter. A data processor is connected to the output of the A/D converter and is arranged to perform the steps of: sampling the value of said output voltage (as represented by the output of the AID converter) a predetermined number of times for an integral number (which may be one) of complete revolutions of one of the rollers when no sheet is passing between the rollers, the diameter of this roller being equal to or a multiple of the diameter of the other roller; storing a first digital value representative of the sum of the values sampled in the last-mentioned step; sampling the value of said output voltage said predetermined number of times for an integral number of complete revolutions of said one of the rollers when an item comprising a single or multiple sheet is passing between the rollers; storing a second digital value representative of the sum of the values sampled in the last-mentioned step; and subtracting the first digital value from the second digital value to produce a third digital value on the basis of which a determination is made as to whether a single or multiple sheet has passed between the rollers.
As mentioned in the above-identified document, an advantage of this apparatus is that by virtue of subtracting said first digital value (stored when no sheet is passing between the rollers) from said second digital value (so as to produce said third digital value) possible problems due to roller noise are eliminated. By roller noise is meant variations in the output of said voltage generator brought about by various factors such as bearing wear and tolerances, dirt on the rollers and roller eccentricity.
A limitation of the known apparatus referred to above is that the spacing between the leading edges of successive items fed to the apparatus must be at least as great as the circumference of the larger of the rollers (or at least as great as the circumference of each roller if they are of the same side). Another limitation is that any divert mechanism positioned in said feed path downstream of said rollers must be spaced from the nip of the rollers by a distance at least equal to said circumference.